This invention relates to plasma induced decomposition of silicon-containing cyclobutanes to form continuous coatings coatings have useful properties such as corrosion resistance and abrasion resistance and are highly electrically insulating.
A number of chemical vapors are known to be useful for forming films or coatings by plasma induced reactions and corresponding deposition onto a substrate. For example, it is known to produce films containing silicon and carbon by the plasma induced reaction of mixtures of silane (SiH.sub.4) and a hydrocarbon such as methane or ethane. The films produced by such processes often contain residual hydrogen as well as silicon and carbon and may be referred to as "hydrogenated silicon carbide". Such films are also commonly described as "amorphous silicon carbide" and may also contain an excess of either carbon or silicon relative to the stoichiometric proportion.
Hydrogenated silicon carbide films find their principal application in photovoltaic devices, where they serve as relatively large band gap "windows" in the top layer of a solar cell. Their advantages for this use include a relatively low and adjustable absorption of visible radiation and the ability for doped material to make a good ohmic contact with the collection electrodes.
One problem associated with formation of hydrogenated silicon carbide films by such methods results from the differing rates of decomposition of silane and hydrocarbons in plasmas. For example, K. Tachibana et al., Symp. Proc. 7th Int. Symp. Plasma Chem., 588-93 (1985), teaches using a large excess of the hydrocarbon in order to overcome the disparity in reactivity of the hydrocarbon relative to silane. Also, silane is a very hazardous chemical which requires the utmost caution and consideration for safety during it use.
Silacyclobutane and disilacyclobutane as well as many of their derivatives are known materials. The thermal decompositions and reactions of these materials have been studied extensively in academic circles because of the substantial theoretical interest in the silene (H.sub.2 Si.dbd.CH.sub.2) and silylene (H.sub.3 CSiH) intermediates produced upon thermolysis of these materials. For example, T. Barton and N. Tillman, J. Am. Chem. Soc. 109, 6711 (1987), describe studies of the flash vacuum pyrolysis of 1,1-dideuteriosilacyclobutane wherein gaseous products of the pyrolysis are identified. R. Conlin and R. Gill, J. Am. Chem. Soc. 105, 618 (1983), report that silacyclobutane decomposes into ethylene, silene, and several silyenes at temperatures above 400.degree. C.
Pola et al., J. Organomet. Chem. 341, C13 (1988), describe a gas-phase reaction of 1-methyl-1-silacyclobutane initiated by infrared radiation from a carbon dioxide laser. These workers observed deposition of a transparent "organosilicon polymer" on cold surfaces and proposed that a gas phase polymerization had occurred. The characteristics of the "organosilicon polymer" described by Pola et al. suggest that it has appreciable organic groups present (particularly methyl groups) so that it is substantially different from the hydrogenated silicon carbide coatings obtained by plasma induced reaction of mixtures of silane and a hydrocarbon as described above.